Page 196 - Phase-Locked Loops Design, Simulation, and Applications

MIXED-SIGNAL PLL APPLICATIONS PART 1: INTEGER-N FREQUENCY
SYNTHESIZERS Ronald E. Best 121

Figure 6.2 A system equal in performance to Fig. 6.1 but with an additional reference divider
that makes it possible to use a higher-frequency reference, normally a quartz
oscillator.

phase detectors, frequency dividers, and so on, as indicated by the dashed enclosure in Fig.
6.2. Due to its low power consumption, high noise immunity, and large range of supply
voltages, CMOS is the preferred technology today. The limited speed of CMOS devices
precludes their application for directly generating frequencies in the range of 100 MHz or
more (at least at the time of this writing).

Integer-N Frequency Synthesizers with Prescalers

Fixed division ratio prescalers

To generate higher frequencies, prescalers are used; these are often built with other IC
technologies such as ECL, Schottky TTL, GaAs (gallium-arsenide), or SiGe (silicon-
germanium compound) (see Fig. 6.3). Such prescalers extend the range of frequencies into the

microwave frequency bands. 7,37